1. Field of the Invention
The present invention relates to a bias circuit that operates relatively independent of operating temperature. More specifically, the present invention relates to a bias circuit for biasing a transmitter amplifier stage, wherein the biasing is substantially independent of the operating temperature.
2. Related Art
FIG. 1 is a block diagram of a portion of a conventional transmitter power amplifier circuit 100. Transmitter power amplifier circuit 100 is typically used in a cellular telephone handset. Transmitter power amplifier circuit 100 is used to amplify a radio frequency signal for transmission from the handset to a nearby receiving station. Transmitter power amplifier circuit 100 includes a bias circuit 101 and a transmitter amplifier stage 102. Bias circuit 101 is described in more detail in U.S. Pat. No. 6,441,687.
Bias circuit 101 includes diode element 110, resistors 111-113, NPN bipolar transistors 121-122, and nodes 104-105, which are connected as illustrated. Transmitter stage 102 includes resistors 1301-130N and NPN bipolar transistors 1311-131N, which are connected as illustrated.
In general, transistor 121 operates as a reference device. Bias circuit 101 causes a collector current I3 to flow through transistor 121. This collector current I3 is reflected to transistors 1311-131N, thereby causing corresponding collector currents IC1, IC2, . . . ICN to flow through these transistors 1311-131N. A DC voltage (not shown) is applied to the collectors of transistors 1311-131N Resistor 111 provides most of the current flowing through transistor 121. However, some of the current through transistor 121 is supplied by resistor 112 and diode 110. Resistor 112 and diode 110 form a level-shifter, which provides a voltage to the base of transistor 122. Transistor 122 operates as an emitter-follower to supply base current to transistors 121 and 1311-131N. The voltage at the emitter of transistor 122 is provided to the base of transistor 121 through resistor 113, thereby completing a feedback loop that sets the operating point of bias circuit 101.
More specifically, bias circuit 101 operates in the following manner. A supply voltage VCC (e.g., 3.3 Volts) is applied to the collector of transistor 122, and a reference voltage VREF is applied to resistors 111 and 112. The reference voltage VREF is typically a regulated voltage (e.g., 2.8 V±0.1 V) received from a constant voltage source (not shown), such as a band-gap referenced voltage regulator.
The voltage (V112) across resistor 112 is defined as follows:V112=VREF−(VBE1+VBE2)  (1)where VBE1 is equal to the base-to-emitter voltage of transistor 121, and VBE2 is equal to the base-to-emitter voltage of transistor 122.
Resistor 112 has a resistance of R2. The current (I2) flowing through resistor 112 is therefore defined as follows:I2=V112/R2=(VREF−(VBE1+VBE2))/R2  (2)
Because resistor 111 is connected in parallel with resistor 112 and diode 110, the voltage across resistor 111 (V111) is equal to the voltage across resistor 112 (V112) plus the voltage across diode 110 (VD1). Resistor 111 has a resistance of R1. The current (I1) flowing through resistor 111 can therefore be defined as follows:I1=V111/R1  (3)I1=(V112+VD1)/R1  (4)I1=(VREF−(VBE1+VBE2)+VD1)/R1  (5)
Assuming that the base current of transistor 122 is negligible, the collector current (I3) flowing through transistor 121 is equal to I1+I2. Thus, the collector current I3 can be defined as follows.I3=(VREF−(VBE1+VBE2))/R2+(VREF−(VBE1+VBE2)+VD1)/R1  (6)
The bases of transistors 121 and 1311-131N are all biased by the voltage (VBIAS) on node 105. Thus, the collector current I3 of transistor 121 is proportional to the collector currents IC1-ICN of transistors 1311-131N. In this manner, bias circuit 101 selects the collector (DC bias) currents in the transistors 1311-131N of transmitter stage 102.
The bases of transistors 1311-131N are also connected to receive radio frequency (RF) input signals IN1-INN, respectively. Transistors 1311-131N provide amplified RF output signals OUT1-OUTN in response to the input signals IN1-INN and the bias voltage (VBIAS) on node 105.
It is desirable for the collector currents in transistors 1311-131N to be constant with respect to varying temperature. Variations in these collector currents undesirably result in variations in the power of the output signals OUT1-OUTN. In order for the collector currents IC1-ICN of transistors 1311-131N to be constant with respect to temperature, the collector current I3 must be constant with respect to temperature. However, as described below, the collector current I3 is not constant with respect to temperature. The voltage across a PN semiconductor junction (diode) decreases as the temperature of the junction increases. Thus, as the temperature of bias circuit 101 increases, the (junction) voltages VD1, VBE1 and VBE2 all decrease. As defined by Equation (6), as the voltages VD1, VBE1 and VBE2 decrease, the collector current I3 increases. As a result, the collector currents IC1-ICN through transistors 1311-131N similarly increase. The increased collector currents through transistors 1311-131N undesirably change the operating characteristics of transmitter stage 102. More specifically, the increased collector currents in transistors 1311-131N can undesirably lower the power efficiency of transmitter power amplifier stage 102. Similarly, decreases in temperature will result in decreased collector currents through transistors 1311-131N, thereby undesirably reducing the power gain of transmitter power amplifier stage 102.
Transmitter power amplifier circuit 100 is typically used in cellular telephone handsets, which are typically required to operate within an extreme range of temperatures (e.g., −30° C. to 85° C.). As the temperature changes, the operating characteristics of bias circuit 101 will change, such that the bias voltage for the transmitter circuit will vary, thereby resulting in considerable variations in the output power gain and power efficiency of the transmitter power amplifier circuit 100.
It would therefore be desirable to have a bias circuit for a power amplifier stage that is substantially independent of temperature. It would also be desirable to have a bias circuit for a power amplifier stage that allows for a selectable relationship between bias current and temperature.